1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission for a vehicle, and more particularly to a downhill brake downshift control apparatus for an automatic transmission for a vehicle.
2. Description of the Related Art
Various control apparatuses for an automatic transmission for a vehicle have been proposed. In general, a plurality of kinds of shift maps for providing various shift characteristics are prepared, and a value indicative of a running resistance is obtained from an acceleration. Then, it is determined whether the vehicle running road is a level road, an uphill road, or a downhill road according to the value indicative of the running resistance, and any one of the plural shift maps is selected according to the kind of the vehicle running road determined above. Then, a gear position is controlled according to the selected shift map. In other words, an expected acceleration preliminarily set according to an engine output is compared with an actual acceleration obtained in actual, and the slope of a road surface on which the vehicle is running is determined from the result of this comparison. Then, an optimum shift map is selected according to the road surface slope determined above, and a gear position is decided according to the selected shift map.
For example, Japanese Patent No. 2959938 discloses a control apparatus for an automatic transmission for a vehicle such that forcible downshift in running of a vehicle on a downhill road is suppressed more as the road surface slope becomes greater unless the driver's intention to decelerate the vehicle is strong, thus improving the drivability. Further, Japanese Patent No. 2862538 discloses a control apparatus for an automatic transmission such that when the road surface slope is detected to be a downhill slope greater than a predetermined value, a brake operation is detected, and a deceleration greater than a predetermined value is detected, the normal shift characteristic is changed to a shift characteristic such that a shift line in a low load condition is shifted to a higher vehicle speed, thereby preventing troublesome shift due to an accelerator operation.
In such a conventional control apparatus for an automatic transmission, a downshift vehicle speed is changed by changing the shift map to a shift map for a downhill road, and it is determined whether or not downshift is to be performed according to the downshift vehicle speed changed above. Accordingly, there is a case that desired downshift is not performed. Specifically, there is a problem such that downshift occurs with a delay from a brake operation, that a plurality of steps of downshift occur against the driver's intention, or that downshift does not occur in a high deceleration region where downshift is to be performed.